Best of the ISP-Lists

IP Conflict

Members of the ISP-Tech list discuss the many possible causes of IP conflict.

[November 20, 2000]

On the ISP-Tech list in November, JB inquired,

"One of our web servers sent a message saying the TCP/IP services were stopped because of a conflict of IPs with another equipment or service, despite the fact that no new IP was assigned in the last 72 hours. We did a trace with the faulty IP and found that it was assigned to a dialup user. Is it possible that the user could be validated, assigned an IP from the pool, and then change it 'on the air'?"

EK contended that the situation just doesn't make sense:

"I don't really see how a dialup user could set an IP address and actually interfere with one of your servers." A couple of respondents recalled similar problems in the past:

[RW offered] "I've had this happen—it's a Macintosh problem. If the customer hard codes the settings in a Mac, the dialup settings will not override it.

We need equipment manufacturers to allow us to block people from doing this."

[JB advised] "I have had problems with Windows 95 and 98 machines where dialup networking does not release the IP address assigned upon disconnection. When attempting to reconnect, it tries to use the old IP address, which may already be in use. If the connection is made to the same box where that IP is assigned from, the duplicate IP connection will terminate after authentication. The fix is to upgrade dialup networking, or reboot to get the machine to clear the previously assigned IP address.

If that doesn't work, have your customer delete dialup networking and re-install."

Others looked at the situation from other angles:

[TB explained] "You must be allowing client-assigned IP addresses. That customer specified an address in his dialup networking settings, as opposed to using the server-assigned IP address."

[PF countered] "There is no real way for him to request or change an IP address. Make sure your IP pool is large enough for all modems, plus a few extra.

If you have ten modems, use at least a dozen IPs so they round-robin and a 'fresh' or 'least used' IP is assigned."

—End